It is difficult to build directional antennas for borehole radar due to the limited space available in the borehole. Recently published results show, however, that it is possible to obtain directional information even from electrically small antennas. In this paper, we use Finite-Difference Time-Domain (FDTD) modelling to optimise the design of a four-element directional antenna array, so that it provides optimal directional discrimination for a 250 MHz radar system. Modelling is used to investigate the use of different filler materials inside the antenna array and the effect of mutual coupling on the directional antenna array. The effect of the borehole material (i.e. air or water) on the directionality of the antennas is also explored. Lastly, the effect of unevenly distributed borehole material around a borehole radar system, with a diameter that is much smaller than the diameter of the borehole, is discussed. A filler material that matches the permittivity of the surrounding rock offers a good compromise between the permittivity of air and that of water. The borehole contents do not affect the ability of an antenna array to determine direc-tion, but the antenna must be placed centrally in the borehole, so the use of centralizers is recommended.